Dedicated communications systems that use communications cables and plug and jack connectors are commonly employed to enable computers, servers, printers, facsimile machines and other electronic devices to communicate with each other, through a private network, and with remote locations via a telecommunications service provider. Such communications system may be hard wired through, for example, the walls and/or ceilings of a building. Individual jacks such as RJ-45 style modular wall jacks are mounted in offices throughout the building. The communications cables provide a communications path from these jacks to network equipment (e.g., network servers, switches, etc.) that may be located in a computer room. Communications cables from external telecommunication service providers may also terminate within the computer room.
In the above-described communications systems, the communications cables that are connected to end devices are typically terminated into one or more communications patching systems that may simplify later connectivity changes. These communications patching systems typically include a plurality of “patch panels” that are mounted on one or more equipment racks. As is known to those of skill in the art, a “patch panel” refers to an inter-connection device that includes a plurality of connector ports (e.g., RJ-45 jacks) on a front side thereof. Each connector port is configured to receive a first communications cable that is terminated with a mating connector (e.g., an RJ-45 plug). Typically, a second communications cable is terminated into the reverse side of each connector port. With respect to a jack on an RJ-45 patch panels, the second communications cable is typically terminated into the reverse side of the patch panel by terminating the individual conductors of the cable into corresponding insulation displacement contacts or other wire connection terminals of the jack. Each connector port on the patch panel may provide communications paths between the first communications cable that is plugged into the front side of the connector port and the second communications cable that is terminated into the reverse side of the connector port.
FIG. 1 is a simplified example illustrating one way in which a communications patching system may be used to connect a computer (or other end device) 10 located in an office 4 of a building to network equipment 52, 54 located in a computer room 2 of the building. As shown in FIG. 1, the computer 10 is connected by a patch cord assembly 11 to a modular wall jack 20 that is mounted in a wall plate 16 in office 4. The patch cord assembly 11 comprises a communications cable 12 that contains a plurality of individual conductors and plugs 13, 14 that are attached to the respective ends of the cable 12. The plug 13 is inserted into a jack (not pictured in FIG. 1) that is provided in the computer 10, and the plug 14 inserts into a plug aperture 21 in the front side of the jack 20. The contacts or “blades” of plug 14 (which are exposed through the slots 15 on the top and front surfaces of plug 14) mate with respective contacts (not visible in FIG. 1) of the jack 20 when the plug 14 is inserted into the plug aperture 21. The blades of plug 13 similarly mate with respective contacts of the jack that is provided in the computer 10.
The jack 20 includes a back-end wire connection assembly 22 that receives and holds conductors from a communications cable 25. As shown in FIG. 1, each conductor of cable 25 is individually pressed into a respective one of a plurality of slots provided in the back-end connection assembly 22 to establish mechanical and electrical connection between each conductor of cable 25 and the jack 20. The communications cable 25 is routed from the back end of the wall jack 20 through, for example, the walls and/or ceiling of the building, to the computer room 2. As there may be hundreds or thousands of wall jacks 20 within an office building, a large number of cables 25 may be routed into the computer room 2.
A first equipment rack 30 is provided in the computer room 2. A plurality of patch panels 32 are mounted on the first equipment rack 30. Each patch panel 32 includes a plurality of connector ports 34 such as, for example, modular RJ-45 jacks. Each cable 25 that provides connectivity between the computer room 2 and the various offices 4 in the building is terminated onto the back end of one of the connector ports 34 of one of the patch panels 32. A second equipment rack 40 is also provided in the computer room 2. A plurality of patch panels 42 that include connector ports 44 are mounted on the second equipment rack 40. A first set of patch cords 46 (only two exemplary patch cords 46 are illustrated in FIG. 1) are used to interconnect the connector ports 34 on the patch panels 32 to respective ones of the connector ports 44 on the patch panels 42.
As is further shown in FIG. 1, network devices such as, for example, one or more network switches 52 and network routers and/or servers 54 are mounted on a third equipment rack 50. Each of the switches 52 may include a plurality of connector ports 53. A second set of patch cords 60 connect the connector ports 53 on the switches 52 to the back end of respective ones of the connector ports 44 on the patch panels 42. A third set of patch cords 64 may be used to interconnect other of the connector ports 53 on the switches 52 with connector ports 55 provided on the network routers/servers 54. In order to simplify FIG. 1, only a single patch cord 60 and a single patch cord 64 are shown. One or more external communications lines 66 may be connected to, for example, one or more of the network devices 54 (either directly or through a patch panel). The communications patching system of FIG. 1 thus may be used to connect each computer 10 and the like located throughout the building to the network routers and servers 54 and/or the external communications lines 66 through the network switches 52.
Typically, the information signals transmitted between networked devices (e.g., computer 10 and network server 54) are transmitted over a pair of conductors (hereinafter a “differential pair” or simply a “pair”) rather than over a single conductor. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors of the pair. When signals are transmitted over a conductor in a cable, electrical noise from external sources such as lightning, electronic equipment, radio stations, etc. may be picked up by the conductor that degrade the quality of the information signal. When the signal is transmitted over a differential pair of conductors, each conductor in the differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair; thus, the noise signal is cancelled out by the subtraction process.
The cables and connectors in most high speed communications systems include eight conductors that are arranged as four differential pairs. The cascaded plugs, jacks and cabling segments shown in FIG. 1 that provide connectivity between two end devices (e.g., computer 10 and network server 54) is referred to herein as a “channel.” Thus, in most high speed communications systems, a “channel” includes four differential pairs, as four differential pairs are typically provided in the cabling and connectors that are used to interconnect the two devices. Typically, the conductors in the communications cables and the contacting structures within communications connectors are located in close proximity to each other. As a result, energy from a signal that is transmitted over a first differential pair of the channel may capacitively and/or inductively couple to one or more of the other differential pairs. This capacitive and inductive coupling gives rise to another type of noise that is called “crosstalk.”
More specifically, “crosstalk” refers to unwanted signal energy that is induced onto the conductors of a first “victim” differential pair from a signal that is transmitted over a second “disturbing” differential pair. The induced crosstalk may include both near-end crosstalk (“NEXT”), which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), and far-end crosstalk (“FEXT”), which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both types of crosstalk comprise an undesirable noise signal that interferes with the information signal on the victim differential pair.
Crosstalk that arises between two differential pairs that are part of the same channel is typically referred to as “internal” crosstalk. Because communications cables are often bundled together for routing through the walls, floors and/or ceilings of buildings and/or because communications connectors are often located in very close proximity to each other in, for example, patch panels and switches, crosstalk may also occur between one or more differential pairs of a first channel and one or more differential pairs of a second channel. Such crosstalk between differential pairs of different channels is typically referred to as “alien” crosstalk.
A variety of techniques may be used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor of a first differential pair in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs in the cable. Additionally, jacks and plugs have been developed that include crosstalk compensation circuits that introduce compensating crosstalk that is used to cancel much of the “offending” crosstalk that is unavoidably generated in many industry-standardized plug and jack designs.